Evaluating And Avoiding Bicycle Wheel Rim Wear From Braking

By Jim Langley

This week I cover an issue that almost any roadie can run into if they log enough miles: rim wear.

There’s an “almost” in that sentence because disc-brake equipped roadsters have hit the shops in force, and you might already be enjoying their advantages. One of the best being zero rim wear from braking. If you’ve switched to a machine with disc brakes, you’re dismissed and can hit the road — or gravel!

Disc trivia:  One of the attractive things about disc brake road bikes is that it’s possible to shave weight at the rims since they don’t need braking tracks. And, the best place to save weight is on rotating components, so this is a performance enhancement you should feel on every ride.

The idea for tips on rim wear comes from my buddy Leo Jed who, when he’s not bicycle touring with the Santa Cruz County Bicycle Club, serves tirelessly to improve cycling here on the Santa Cruz Traffic Safety Coalition and Santa Cruz Regional Transportation Bike Committee.

Leo wrote

“Your RBR features about aging carbon’s reliability reminded me of a question about wheels. Specifically, how do you determine the condition (wear) and therefore safety of a wheel’s rim? I know some rims have wear indicators (dimples or grooves); however, mine do not.

“I’ve looked/felt for a curvature on the rim faces and put a straightedge across the rim’s braking surfaces. I feel a slight curvature, greater on the front rim and a very small gap with a straight edge across the rim. Again, slightly more of a gap on the front than the back.

“I’m riding hand-built wheels with Mavic CXP 33 rims with 36 spokes, tied and soldered. The wheels have stayed true over 35,000 miles without problems. However, with this mileage I’m starting to worry about the possibility of a high-speed rim break and blowout.”

Note:  Leo mentions “tied and soldered” spokes. This is an extra step some wheel builders take, believing it adds strength and improves wheel performance. They carefully and very tightly wrap fine copper wire multiple times around the spoke crosses closest to the rim. Then to keep it in place, they solder the wire. Here’s a photo.

My reply

That’s a great question, Leo. I remember a conversation I had with a customer with a cracked rim when I was a shop service manager.

He was convinced his rim failure was a defect in the aluminum rim even though there was obvious heavy wear and deep scoring. So, I believe other riders may not understand that their rims can wear out simply from braking.

Note:  In the past, most carbon rims had aluminum braking tracks, which can wear, too. Newer wheels usually have carbon braking surfaces, so carbon-compatible composite brake pads are required to prevent wear and to provide sufficient friction for good braking.

Putting a straightedge on the rim sidewall like you’re doing, Leo, is a great way to check for wear. The brake pads only contact one area, so you usually have the untouched rim surface above and below that area. If you hold the straightedge just right and then use a second ruler as a depth gauge, you can measure the deepest point of the wear track.

What’s difficult to determine is how thick the rim wall was when it was new. That’s not a dimension the rim makers usually provide. Now, you could try to find the same rim and measure it, but that may or may not be practical or possible. A good bike shop with the rim in stock might be able to help, but again, not every shop has all rims.

So, what I would do is see if you can measure a part of the rim side that’s not been braked on. You’ll need to let the air out a tire to do this. You just need to use an outside caliper and get it in there to give you a reading on the thickness of an untouched part of the rim wall. The rim walls are the same wall thickness from bottom to top usually.

Once you have that, you now can put an approximate number on what you had in material when the rim was new and also how much has been worn away through braking. (The full measurement minus the depth measurement.)

In my experience, the rim will hang in there a long, long time, probably down to 25 percent of its original width, or even more. So, if it’s 2mm wide wall thickness, you could remove up to 1.5 mm in thickness and I believe it would still be strong enough to hold the tire and keep working. But any less and it’s probably going to fail pretty soon.

The good news is that you usually don’t have a blowout or a crash. What usually happens is that the rim starts to bulge and you feel it when braking and look to see that the rim is splitting at the bulge. I’ve had split rims keep working for a long time. They don’t always fail even though they’re split. But, ideally you would not take any chances.

Avoiding bicycle wheel rim wear

The best way to prevent worn out rims is to check your brake pads frequently for bits of debris from the road that get embedded in them. Once the pads have a few tiny stones in them, the pads become more like sandpaper and start scoring your rims every time you brake.

Every month or so, and after every rainy ride (there’s more debris on the road when it rains), inspect your pads closely and pick out any embedded debris with an awl or pick. It’s also a good idea to replace pads before they get too old and harden, which can also wear the rims.

Leo wrote back

“Several months ago, when the tire was off, I did use a caliper as you suggested. The caliper decided to go on the fritz, though, and I didn’t follow through. I did, however, shine a light behind the straightedge and although light passed under the edge, the rear wheel gap was only about half the front, as we would expect. Good to have a ballpark range of 75% material loss and that a full blowout is unlikely.

“But, I have to tell you that about a year ago I was riding around Paso Robles, California, with a friend when his rim split and blew the tire. The explosion was strong enough to make the rim separate and a piece slammed into the brake and stopped the bike! Fortunately we were riding slowly, enjoying the scenery, and no one crashed. But it scared me and got me thinking about my rims.”

Check your rims and pads

Since there’s a lot of great riding left in the year, let Leo’s story about his friend’s rim separation be a warning to inspect your rims and brake pads. If you know that you have mega miles on your wheels and haven’t checked the rims and/or brake pads in a long time, do it now.

Keep in mind that most modern brakes have cartridge brake pads that make pad replacement quick and easy. So, if your pads are worn and full of debris, it’s usually best to replace them with new ones. I recommend always having a spare set of pads on hand so you’re ready to do this when you need to.

Just loosen each old pad, slide it out, slide in the new one — being careful to put it in in the correct direction (they’re marked) — and tighten it in place. If you used your cable barrel adjustment to tighten your brake as the pads wore, the final step is turning the adjuster back to its original position. If you don’t do this, you might not have enough clearance between your new pads and the rim.